When measuring objects using computed tomography, scattered radiation is undesirable, because it increases the signal background and noise of the detector signals and furthermore produces undesired artefacts in the reconstructed images. The scattered radiation can be reduced if the computed tomography system is designed and/or operated such that only a limited, relevant solid angle range is illuminated with X-rays. If only relevant object details are irradiated, that is to say only those parts of which the image information is actually used, the image quality can be improved.
In microfocus sources which are also operated at high power and therefore with focal spot sizes up to the millimeter range, the used ray angle is generally so large that only part of the X-ray beam is incident on the sensor surface of the detector and another part travels past the sensor surface or is incident next to the sensor surface. The reason for this is that the sensor surface would otherwise be illuminated inhomogeneously. In order to obtain sufficiently homogeneous illumination, it is typical to select used ray angles that are of a size such that the sensor surface is illuminated many times over. In a computed tomography system having a variable distance between X-ray source and detector, the used ray angle should, with a constant size of the sensor surface, be adapted to the distance to reduce scattered radiation.
In this respect, two procedures are known from the related art. In the first procedure, a stop size, i.e., an aperture of the stop, which defines the used ray angle is selected to have a size such that the sensor surface of the detector is completely illuminated even in the case of the largest possible focal spot. The stop is here located at a fixed distance from the focal spot, independently of the size of the focal spot.
The second procedure provides a window collimator, which is adjustable by a motor and which permits the setting of the size of the illuminated region in one or two direction(s) that extend transversely to the propagation direction of the radiation. The stop windows of the collimator are located in a common plane at a distance from the focal spot which is not adaptable to a change in the focal spot size. The sharpness of the imaging of the stop windows in the detector plane therefore depends on the stop size.
Also known from medical technology, in which the radiation exposure of a patient is to be kept as small as possible, are so-called “multi-leaf” collimators. The latter consist of leaves that are adapted for individual recordings to the recording conditions. However, for automated actuation, a great number of independent actuators are used.